1. Field of the Invention
This invention relates to an apparatus for reading out a radiation image stored on a stimulable phosphor sheet. This invention particularly relates to a radiation image read-out apparatus wherein the level of stimulation energy of stimulating rays, which are made to impinge upon the stimulable phosphor sheet, per unit area of the stimulable phosphor sheet is adjustable.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet or simply as a sheet) is first exposed to a radiation passing through an object to have a radiation image of the object stored thereon, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed to reproduce a visible image having an improved quality, particularly a high diagnostic efficiency and accuracy.
FIG. 3 is a schematic view showing an example of a radiation image read-out apparatus employed in the aforesaid radiation image recording and reproducing system.
In the radiation image read-out apparatus of FIG. 3, a laser beam 1a of a predetermined intensity is emitted as stimulating rays from a laser beam source 1 to a galvanometer mirror 2. The laser beam 1a is deflected by the galvanometer mirror 2 and is made to impinge upon a stimulable phosphor sheet 3 positioned below the galvanometer mirror 2 so that the sheet 3 is scanned by the laser beam 1a in the main scanning direction, i.e. in the width direction of the sheet 3 as indicated by the arrow A. While the laser beam 1a impinges upon the stimulable phosphor sheet 3, the sheet 3 is conveyed in the sub-scanning direction as indicated by the arrow B, for example, by an endless belt device 9. Therefore, scanning in the main scanning direction is repeated at an angle approximately normal to the sub-scanning direction, and the whole surface of the stimulable phosphor sheet 3 is two-dimensionally scanned by the laser beam 1a.
As the stimulable phosphor sheet 3 is scanned by the laser beam 1a, the portion of the sheet 3 exposed to the laser beam 1a emits light having an intensity proportional to the radiation energy stored. The light emitted by the stimulable phosphor sheet 3 enters a transparent light guide member 4 from its light input face 4a positioned close to the sheet 3 in parallel to the main scanning line. The light guide member 4 has a flat-shaped front end portion 4b positioned close to the stimulable phosphor sheet 3 and is shaped gradually into a cylindrical shape towards the rear end side to form an approximately cylindrical rear end portion 4c which is closely contacted with a photomultiplier 5. The light emitted by the stimulable phosphor sheet 3 upon stimulation thereof and entering the light guide member 4 from its light input face 4a is guided inside of the light guide member 4 up to the rear end portion 4c, and received by the photomultiplier 5 via a filter (not shown) for selectively transmitting the light emitted by the stimulable phosphor sheet 3. In the apparatus shown, a light detection means is constituted by the light guide member 4 and the photomultiplier 5. Thus the light emitted by the stimulable phosphor sheet 3 in proportion to the radiation energy stored thereon is detected and converted into an electric image signal by the photomultiplier 5. The electric image signal thus obtained is sent to an image processing circuit 6 and processed therein. The electric image signal thus processed is then reproduced into a visible image and displayed, for example, on a cathode ray tube (CRT) 7, or stored on a magnetic tape 8, or directly recorded as a hard copy on a photographic film or the like.
In the aforesaid radiation image read-out apparatus, when the conditions of the read-out apparatus such as the power and the scanning speed of the stimulating rays are the same, the amount of light emitted by the same stimulable phosphor sheet becomes different in accordance with the radiation dose in the radiation image recording step. Specifically, in the case where the stimulable phosphor sheet is exposed to a large amount of radiation in the radiation image recording step, a high level of radiation energy is stored on the sheet, and therefore the amount of light emitted by the sheet in proportion to the stored radiation energy when it is exposed to stimulating rays becomes large as a whole.
On the other hand, the amount of light emitted by the stimulable phosphor sheet in proportion to the stored radiation energy may be changed by changing the level of stimulation energy of stimulating rays per unit area of the sheet. FIG. 4 is a graph showing the relationship between the amount of light emitted by a stimulable phosphor sheet and the stimulation energy per unit area of the sheet. As shown in FIG. 4, the amount of light emitted by the stimulable phosphor sheet becomes large as the level of stimulation energy of stimulating rays per unit area of the sheet becomes high. Also, the ratio of the extent of change in the amount of light emission to the extent of change in the level of stimulation energy of stimulating rays is approximately the same between a sheet portion where the level of stored radiation energy is high (curve A) and a sheet portion where the level of stored radiation energy is low (curve B). Therefore, the contrast of a reproduced visible image is not so much affected even though the level of stimulation energy of stimulating rays to which the stimulable phosphor sheet carrying a radiation image stored thereon is exposed is changed to make the amount of light emitted by the sheet change as a whole. Accordingly, even if image read-out is conducted using a decreased level of stimulation energy per unit area of the sheet, it is still possible to read out the radiation image accurately. In order to decrease the level of stimulation energy of stimulating rays per unit area of the sheet, the scanning speed of the stimulating rays with respect to the sheet may be increased, or the amount of the stimulating rays may be decreased. In the former case, it becomes possible to shorten the time required for image read-out, thereby to increase the read-out efficiency per unit time. In the latter case, it is possible to decrease the running cost of the apparatus by decreasing the power of the stimulating ray source.
On the other hand, in the case where the level of radiation energy stored as image information on a stimulable phosphor sheet is comparatively low and the amount of light emitted by the sheet is small, it is not advantageous from the viewpoint of noise to decrease the level of stimulation energy of stimulating rays and to make the amount of light emitted by the sheet smaller. FIG. 5 is a graph showing the relationship between the amount of light emitted by a stimulable phosphor sheet and the level of noise. As shown in FIG. 5, the level of noise is high at a region where the amount of light emitted by the stimulable phosphor sheet is small, and gradually decreases as the amount of light emission increases. Also, the extent of decrease in noise becomes gradually small as the amount of light emission increases, and is almost saturated at a region where the amount of light emission is large. Therefore, when stimulation energy of stimulating rays is decreased at a sheet portion where the level of stored radiation energy is high so that the light emission amount I1A decreases to the light emission amount I1B, the level of noise changes from .delta..sup.2 1A to .delta..sup.2 1B and thus noise does not so much increase. However, when stimulation energy of stimulating rays is decreased as a sheet portion where the level of stored radiation energy is low so that the light emission amount 12A decreases to the light emission amount I2B, noise increases markedly from .delta..sup.2 2A to .delta..sup.2 2B. Accordingly, in the case where the level of stimulation energy of stimulating rays per unit area of a stimulable phosphor sheet is changed, it is necessary to approximately ascertain the level of radiation energy stored on the stimulable phosphor sheet, and to adjust the level of stimulation energy of stimulating rays per unit area of the sheet based on the ascertained level of stored radiation energy.